Low temperature separation of gaseous mixtures



Jan. 29, 1957 H. VESQUE 2,779,174

LOW TEMPERATURE SEPARATION OF GASEOUS MIXTURES Filed Oct. 29, 1954 .720 9,72%, Hawe/ KES z/E yffaw LOW TENIPERATURE SEPARATION OF GASEOUS BHXTURES Henri Vesque, Paris, France, assignor to LAir Liquide, Societe Anonyme pour lEtude et lExploitation des Procedes Georges Claude, Paris, France Application October 29, 1954, Serial No. 465,712

Claims priority, application France June 29, 1954 11 Claims. (Cl. 62-1755) In known procedures, air is separated by rectification into nitrogen and oxygen in acceptable degrees of purity, using an apparatus having two rectification columns operating at different pressures and in mutual connection with a heat exchange mechanism, including means acting as a condenser at the top of the high pressure column and as a vaporizer at the base of the low pressure column. In these procedures there has been extracted from the low pressure column a mixture of the major part of the argon contained in the air to be separated, as well as part of at least one of the other principal components, oxygen and nitrogen, and this mixture has been rectified in an auxiliary column, operating at a I pressure in the neighbourhood of that of the low pressure column of the principal assembly. The rectification carried out in the auxiliary colmnn yields a concentrated fraction of argon, which can then be purified by chemical or physical processes.

This fraction is generally a mixture made up essentially cause the argon, scarcely more volatile than the oxygen, 7

to leave the top of the auxiliary column relatively free 011 oxygen.

To obtain this reflux, various solutions have been proposed as, for example, the use of a cooling cycle fed either by argon or by nitrogen. Another solution, simpler, but leading to inferior yields, consists in drawing in gaseous state, a part of the nitrogen produced by the principal column at the greatest pressure, in liquefying this nitrogen in an exchanger placed at the base of the auxiliary column, and vapourizing it, after expanding it in an exchanger placed at the top of the auxiliary column. The quantity of nitrogen thus taken from the main column is consequently lacking at the top of the low pressure column, where the required reflux is provided by nitrogen drawn in the liquid state from the high pressure column, then expanded. Only a limited quantity of this nitrogen can be disposed of to assure operation of the auxiliary column, if the operation of the main column is not to be interfered with. The yield of of the argon fraction obtainedis low.

nited States Patent ice The present invention permits on the contrary the.

rangement, on one hand of two columns functioning at different pressures and mutually connected with a heat exchange apparatus forming a condenserat the top of the highest pressure column and a vaporizer at the base of the lowest pressure column, and on the other hand an auxiliary column fed by an intermediate fraction drawn from thelowest pressure.;column and-containing all or a part of the argon of the air to be separated.

These three columns are hereafter called respectively, Y

the high pressure column, the low pressure column, and auxiliary column.

The invention embodies the following features. All air to be separated, compressed to a pressure greater than that of the high pressure column, and cooled by exchange with the products of its separation to a temperature appreciably higher than its temperature of liquefaction under the said pressure, is divided into two parts. One of these parts is introduced into the high pressure column withan expansion elfected, preferably with performance of external work, to the pressure of this column. The other part is cooled to the temperature of liquefaction, and partially liquefied by heat exchange pressure column. The liquid thus obtained is used, after expansion, to cause by its vaporization, cooling by in direct contact with the upper part of the auxiliary column Optionally, the liquid from the coil at the bottom of the high pressure column may first be passed through a coil at the bottom .of the auxiliary column. The gas resulting from this vaporization is introduced into the low pressure column.

There is thus extracted in the apparatus, on the one hand argon at the top of the auxiliary column, and on the other hand nitrogen and oxygen respectively at the top and at the bottom of the low pressure column. I

.The following example, which is not intended to be limiting, illustrated by reference to the attached drawing of a preferred apparatus, will make the principle of the invention better understood.

The air to be separated into oxygen, nitrogen and argon, previously freed byknown methods of entrained humidity and carbon dioxide and compressed to about 15 atmospheres and at ambient temperature enters at 1 a first heat exchanger z coole d by separated gases leaving in the neighbourhood of atmospheric pressure. Thus cooled to around l40 C. in the exchanger 2, the air is" divided into two parts. A first part representing approximately 70% of the'air to be separated is expanded by the turbine 4 to a pressure of approximately five atmospheres and enters'atgthis pressure, by the tube 5, intol the high pressure column 10, several trays above the-bottom; The second part is cooled in a heat exchanger 3 to a temperature .of about -158 C., and enters by the tube '6' into'a'coil 7 located a-tthe lower part of the column 10.

The'column 10, according to the usual arrangement, is surmounted by a column 11 functioning in the neighbourhood of atmospheric pressure. The two columns are thermically connectedby the vapourizer-condenser 9 forming a condenser at the top of the column 10 and a vapourizer at the base of the column 11.

Part of the air to be separated, introduced into the column 10 and'formingfas already stated, approximately 70% of'the totalamount of the starting air, is rectified in' this column yielding on the one hand nitrogen which is condensed in total to the liquid state at the top of the column in the condenser 9, and on the other hand a liquid containing all the oxygen and the argon of the air, and having an oxygen content of approximately 50%. One part of the condensed nitrogen and all the liquid rich in oxygen leaves the column 10 respectively by the tubes 12 and 8 to enter the low pressure column 11. The condensed nitrogen is super-cooled in the exchanger 13 then expanded by the valve 14 and enters the 'top of the column 1 1, where it is used as a washing liquid. The liquid rich in oxygen is super-cooled in the exchanger 15, ex panded by the valve 16 and enters the column 11 halfway up the latter.

The column 11 also receives, as will be explained later, the fraction of the air to be separated, not introduced into the high pressure column, and which is liquefied in the coil 7. This fraction having about 30% of the total of the air, enters the column 11 in a gaseous state by the tube 17.

The following products are extracted by the column 11:

(a) At the top by the tube 18, gaseous nitrogen,

(b) At the bottom by the tube 19, gaseous oxygen,

() Laterally, by the tube 20, a gaseous fraction formed of about argon and 90% oxygen with only traces of nitrogen.

This last fraction is introduced into the auxiliary column 21 near the base. The column 21 functions at the same pressure as the column 11. The column 21 is heated at the bottom on the one hand by a gaseous fraction having its origin in the column 11 (stream and on the other hand optionally by a coil 22 into which there passes the air liquefied previously in the coil 7 and having been preferably subjected to sub-cooling of several degrees in the exchanger 26 cooled by oxygen drawn from the column -11. After its passage into the coil 22, this air is expanded by the valve 23 to the pressure of the column 11. It is vapourized in the coil 24, located at the top of the column, causing by this vapourization the formation of the required reflux, then it is introduced into the tube 17 in the col-umn 11, substantially at the same height as the liquid rich in oxygen arrives by the tube 8. From the top of the auxiliary column 21 there is drawn a gaseous fraction containing about 90% argon of the air to be separated, and not having more impurities than about 3% oxygen and about .5 nitrogen. From the base of the auxiilary column 21 there flows a liquid made up of oxygen and argon which returns by the tube 25 to the column 11.

The oxygen drawn in gaseous state from the column 11 is preferably used to cool, in the exchanger 26, the air under pressure of approximately 15 atmospheres which has been liquefied in the coil 7, and then is caused to leave by the conduit 32 after having been reheated in the exchanger 3 and 2 in counter-current with the 'entering air. Alternatively, the exchanger 26 may be omitted and the oxygen (stream 19) carried directly from the column 11 to the exchanger 3.

The gaseous nitrogen leaving the top of the column 11 cools successively, in the exchanger 13 and 15, the liquid nitrogen and the liquid rich in oxygen, all having their origin in the col-umn 10, then leaves by the tube 31 after being reheated, as is the oxygen, in the exchangers 3 and 2.

The argon leaving the top of the column 21 by the tube 27 is equally reheated in the same exchangers and leaves by the tube 33.

Since oxygen usually contains hydrocarbon impurities, it is preferable to include filtration means to remove these impurities when the oxygen is in the liquid state. One set of filters 41, 42 is shown between the coil 7 and the heat exchanger 26. Another set of filters 43, 44 is shown in the pipe line 8. Two filters are shown in each case so that one can be operated while the other is regenerated. These filters are preferably of the type in which the impurities are adsorbed on a solid adsorbent.

The comparison of the apparatus described above, with known apparatuses makes the following points clear. The yield in argon separated is better. The argon fraction is substantially pure. The oxygen obtained is substantially pure.

In the usual systems, there is generally introduced the major part of the air, if not all, in the gaseous state at the base of the high pressure column. Consequently, the oxygen is furnished in the form of a liquid of which the ox gen content is at a maximum, the attendant content of the liquid in equilibrium with atmospheric air at 21% oxygen, an amount which is, to be more exact, about 40% under a pressure of 5 atmospheres.

In the apparatus described above, the portion of the air to be separated which enters the high pressure column enters into it several trays above the bottom, this bottom being heated in indirect contact by condensation in the coil 7 of the other portion of the air. The liquid leaving the column 10 by the tube 8 can thus have an amount of oxygen greater than its usual amount, for example, about 50%. Containing more oxygen, this liquid contains less nitrogen. The latter leaves then at the top of the high pressure column in greater quantity. As a result of this it is possible, even though only a part of the air is separated in the high pressure column, for substantially the same quantity of liquid nitrogen to leave the column as in the usual process where all the air enters the high pressure column. The higher part of the low pressure column receiving then the same quantity of washing liquid at the same purity, functions according to normal conditions and furnishes gaseous nitrogen of the usual purity. On the other hand the lower part of the low pressure column being heated by condensation of the same quantity of nitrogen, the yield at the base of this column is equally satisfactory.

The oxygen obtained will be all the more pure since almost the total amount of argon leaves the column 11 by the tube 20, the oxygen being entrained with the argon returning to the column 11 by the tube 25.

Considering that a high proportion (about 30%) of the air to be separated does not pass through the expan- .sion turbine 4, the lowering of the refrigeration which results from it, should be compensated for by a rise of the initial air pressure, a pressure that is around 15 atmospheres where 12 atmospheres suffices for the conventional type of apparatus.

I claim:

1. A process for the separation by liquefaction and rectification, of a gaseous mixture having three principal constituents, utilizing a main column made up of two rectification zones in mutual heat exchange relationship and under different pressures, and an auxiliary column receiving a fraction having its origin in the lower pressure rectification zone and in which a part of the mixture to be separated previously compressed to a pressure higher than that of the higher pressure rectification zone is expanded to the higher pressure and introduced into the higher pressure zone, comprising liquefying another part of the compressed gaseous mixture by indirect heat exchange with a liquid from the higher pressure rectification zone, then utilizing said other part for cooling by indirect contact With the auxiliary column and introducing said other part into the lower pressure zone.

2. A process according to claim 1 in which part of the gaseous mixture under pressure which has been liquefied by indirect heat exchange with a liquid from the high pressure rectification zone is first utilized for heating by indirect contact with the lower part of the auxiliary column, before serving for cooling the upper part of the auxiliary column.

3. A process according to claim 1 in which the said. other part is sub-cooled by a gaseous fraction from the low pressure column prior to its use for cooling the auxiliary column by indirect contact.

4. A process according to claim 1 in which the said other part is sub-cooled by a gaseous fraction from the low pressure chamber prior to its use for cooling the auxiliary column by indirect contact, and in which the said other part is then utilized for heating by indirect contact with the lower part of the auxiliary column before serving for cooling the upper part of the auxiliary column.

5. A process according to claim 1 in which the gaseous mixture is air and the three principal constituents are oxygen, nitrogen, and argon.

6. A process according to claim 1 in which the gaseous mixture is air and the other constituents are air, nitrogen, and argon and in which part of the oxygen which has been liquefied by indirect heat exchange with a liquid from the high pressure rectification zone is first utilized for heating by indirect contact with the lower part of the auxiliary column, before serving for cooling the upper part of the auxiliary column.

7. A process for the separation by liquefaction and rectification of a gaseous mixture having high, low and intermediate equilibrium constants and utilizing a main column made up of two rectification zones in mutual heat exchange relationship and under different pressures, and an auxiliary column receiving a fraction having its origin in the lower pressure rectification zone and in which a part of the mixture to be separated previously compressed to a pressure higher than that of the higher pressure rectification zone is expanded to the higher pres sure and introduced into the higher pressure zone, comprising liquefying another part of the gaseous mixture by indirect heat exchange with a liquid from the higher pressure rectification zone, then utilizing said other part for cooling by indirect contact with the auxiliary column thereby to concentrate the constituent of intermediate vapor pressure, and simultaneously vaporizing at least in part said other part.

8. A process according to claim 7 in which the gaseous mixture is air and the three constituents are oxygen, argon and nitrogen.

9. A process according to claim 7, in which part of the gaseous mixture under pressure which has been liquefied by indirect heat exchange with a liquid from the higher pressure rectification zone is first utilized for heating by indirect contact with the lower part of the auxiliary column, before serving for cooling in the auxiliary column.

10. A process according to claim 7, in which the said other part is sub-cooled by a fraction from the low pressure zone prior to its use for cooling in the auxiliary column.

11. A process according to claim 7, in which the said other part is sub-cooled by a fraction from the low pressure zone prior to its use for cooling in the auxiliary column, and in which the said other part is then utilized for heating by indirect contact with the lower part of the auxiliary column before serving for cooling in the auxiliary column.

References Cited in the file of this patent UNITED STATES PATENTS 2,209,748 Schlitt July 30, 1940 2,409,458 Van Nuys Oct. 15, 1946 2,433,536 Van Nuys Dec. 30, 1947 2,547,177 Simpson Apr. 3, 1951 2,626,510 Schilling Jan. 27, 1953 

1. A PROCESS FOR THE SEPARATION BY LIQUEFACTION AND RECTIFICATION, OF A GASEOUS MIXTURE HAVING THREE PRINCIPAL CONSITUENTS, UTILIZING A MAIN COLUMN MADE UP OF TWO RECTIFICATION ZONES IN MUTAL HEAT EXCHANGE RELATIONSHIP AND UNDER DIFFERENT PRESSURES, AND AN AUXILIARY COLUMN RECEIVING A FRACTION HAVING ITS ORIGIN IN THE LOWER PRESSURE RECTIFICATION ZONE AND IN WHICH A PART OF THE MIXTURE TO BE SEPARATED PREVIOUSLY COMPRESSED TO A PRESSURE HIGHERE THAN THAT OF THE HIGHER PRESSURE RECTIFICATION ZONE IS EXPANDED TO THE HIGHER PRESSURE AND INTRODUCED INTO THE HIGHER PRESSURE ZONE, COMPRISING LIQUEFYING ANOTHER PART OF THE COMPRESSED GASEOUS MIXTURE BY INDIRECT HEAT EXCHANGE WITH A LIQUID FROM THE HIGHER PRESSURE RECTIFICATION ZONE, THEN UTILIZING SAID OTHER PART FOR COOLING BY INDIRECT CONTACT WITH THE AUXILIARY COLUMN AND INTRODUCING SAID OTHER PART INTO THE LOWER PRESSURE ZONE. 